From Flat Foot to Fat Foot: Structure, Ontogeny, Function, and Evolution of Elephant Sixth Toes

Several groups of tetrapods have expanded sesamoid (small, tendon-anchoring) bones into digit-like structures (predigits), such as pandas' thumbs. Elephants similarly have expanded structures in the fat pads of their fore- and hindfeet, but for three centuries these have been overlooked as mere cartilaginous curiosities. We show that these are indeed massive sesamoids that employ a patchy mode of ossification of a massive cartilaginous precursor and that the predigits act functionally like digits. Further, we reveal clear osteological correlates of predigit joint articulation with the carpals/tarsals that are visible in fossils. Our survey shows that basal proboscideans were relatively flat-footed (plantigrade), whereas early elephantiforms evolved the more derived tip-toed (subunguligrade) morphology, including the predigits and fat pad, of extant elephants. Thus, elephants co-opted sesamoid bones into a role as false digits and used them for support as they changed their foot posture.